Chemical Reactions and Equations — Class 10 Science MCQ with Answers (2026-27)

Updated 23 August 2026 · checked against the CBSE 2026-27 syllabus

12 objective questions on Chemical Reactions and Equations (Class 10 Science — Chemistry), every one checked against the CBSE 2026-27 syllabus. Pick an option and the correct answer is shown straight away, with your marks running at the top. A new question opens each day until all 12 are unlocked.

Chemical Reactions and Equations MCQ questions with answers

1. When steam is passed over red hot iron, iron oxide and hydrogen gas are formed: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
   What is true for the​ balanced​ chemical equation shown above?
   1. Four atoms of water combine with iron to form four atoms of hydrogen.
   2. Three atoms of iron combine with water to form four atoms of hydrogen.
   3. Four molecules of water combine with iron to form an atom of iron oxide.
   4. Three atoms of iron combine with water to form one molecule of iron oxide.

   Answer: d) Three atoms of iron combine with water to form one molecule of iron oxide.

   The equation is 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Read the coefficients: three atoms of iron react with four molecules of water to give one molecule of iron oxide, Fe₃O₄.

2. Cellular respiration is the process by which cells convert glucose to energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
   Carbon dioxide and water are the two new substances​ formed‌ during cellular respiration. What are they known as?
   1. Reactants
   2. Mixtures
   3. Catalysts
   4. Products

   Answer: d) Products

   In any chemical equation the substances written on the right, formed by the reaction, are the products. Glucose and oxygen going in are the reactants.

3. Which of these‌ is‌ an example of decomposition reaction?
   1. Melting of glaciers
   2. Rusting of old bridges
   3. Rotting of fruits and vegetables
   4. Absorption of carbon dioxide by oceans

   Answer: c) Rotting of fruits and vegetables

   Decomposition means one substance breaking down into simpler ones. Rotting breaks complex organic matter into simpler compounds. Melting and dissolving are physical changes, and rusting is a combination reaction.

4. Methane gas released from waste water treatment plants can be used as a source of‌ fuel. Which​ chemical equation represents combustion of methane to produce heat energy?
   1. CH4 + CO2 → 2O2 + 2H2O
   2. CH4 + 2O2 → CO2 + 2H2O
   3. 2O2 + 2H2O → CO2 + CH4
   4. CO2 + 2O2→ CH4 + 2H2O

   Answer: b) CH4 + 2O2 → CO2 + 2H2O

   Combustion means burning in oxygen to give carbon dioxide and water. Only CH₄ + 2O₂ → CO₂ + 2H₂O is balanced and has methane and oxygen as the reactants.

5. A chemical reaction in which two‌ or more substances‌ combine to form a single product is
   1. a decomposition reaction
   2. a combination reaction
   3. a displacement reaction
   4. an exothermic reaction

   Answer: b) a combination reaction

   By definition a combination reaction has two or more reactants joining to give a single product. Decomposition is the reverse, and displacement swaps one element for another.

6. Which of the following reactions‌ is exothermic?
   1. Quick lime is dissolved in water to produce slaked lime.
   2. Calcium carbonate is heated to produce carbon dioxide gas and quick lime.
   3. Lead nitrate is heated to produce lead oxide, nitrogen dioxide and oxygen.
   4. Mercuric oxide is heated to produce mercury and oxygen.

   Answer: a) Quick lime is dissolved in water to produce slaked lime.

   Quick lime dissolving in water gives out a great deal of heat, so it is exothermic. The other three all require continuous heating, which makes them endothermic.

7. For the reaction 3Fe + 4H₂O → Fe₃O₄ + 4H₂, consider these statements: (1) Iron is oxidised. (2) Water is oxidised. (3) Water is the reducing agent. (4) Water is the oxidising agent.
   Which of the statements about the given reaction‌ are correct?
   1. 1 and 4
   2. 2 and 3
   3. 1 and 3
   4. 1 only

   Answer: a) 1 and 4

   In 3Fe + 4H₂O → Fe₃O₄ + 4H₂ iron gains oxygen, so iron is oxidised (statement 1). Water supplies that oxygen and is itself reduced to hydrogen, making it the oxidising agent (statement 4).

8. Calcium oxide can be reduced to calcium by heating with sodium metal. Which compound would act as the‌ oxidising agent in‌ this process?
   1. Sodium
   2. Sodium oxide
   3. Calcium
   4. Calcium oxide

   Answer: d) Calcium oxide

   The oxidising agent is the substance that gives up oxygen and is itself reduced. Calcium oxide loses its oxygen to sodium, so it is the oxidising agent.

9. Manganese dioxide reacts with hydrochloric acid and a gas with bleaching ability is released.
   The chemical reaction​ between​ MnO₂ and HCl is an example of
   1. displacement reaction
   2. combination reaction
   3. redox reaction
   4. decomposition reaction

   Answer: c) redox reaction

   MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O. Manganese is reduced while chloride is oxidised to chlorine, so oxidation and reduction occur together — a redox reaction.

10. Complete combustion of gasoline in a full supply of air: 2C₈H₁₈ + 25O₂ → 16X + 18Y
    Which products X and‌ Y​ are obtained from this reaction?
    1. X = CO₂, Y = H₂O₂
    2. X = H₂O, Y = CO
    3. X = CH₃OH, Y = H₂O
    4. X = CO₂, Y = H₂O

    Answer: d) X = CO₂, Y = H₂O

    Complete combustion of a hydrocarbon in plenty of oxygen always yields carbon dioxide and water, so X = CO₂ and Y = H₂O.

11. Identify the types of chemical​ reaction‌ occurring during the combustion of a fuel.
    1. Oxidation and endothermic reaction
    2. Decomposition and exothermic reaction
    3. Oxidation and exothermic reaction
    4. Combination and endothermic reaction

    Answer: c) Oxidation and exothermic reaction

    Burning combines the fuel with oxygen, which is oxidation, and releases heat, which makes it exothermic. Both descriptions apply at once.

12. A student observed​ a cloud​ of black smoke belching from the exhaust of a moving truck. Choose the correct reason for the black smoke.
    1. Limited supply of air leads to incomplete combustion of fuel.
    2. Rich supply of air leads to complete combustion of fuel.
    3. Rich supply of air leads to a combination reaction.
    4. Limited supply of air leads to complete combustion of fuel.

    Answer: a) Limited supply of air leads to incomplete combustion of fuel.

    Black smoke is unburnt carbon. It appears when there is not enough oxygen for complete combustion, so the fuel burns only partly.